Posada, who was released on Friday pending a hearing on US immigration fraud charges in the town of El Paso, is wanted by both the Cuban and Venezuelan governments who say he is responsible for the bombing of a Cubana Airlines flight between Barbados and Cuba in 1976 which killed all 73 on board.
He was initially jailed for the crime in Venezuela but subsequently escaped and later became involved with CIA operations in Central America.
Reacting to the ruling by Kathleen Cardone, a US district judge, Castro wrote: "He was protected by simply being charged with an immigration violation."
"The response is brutal. The US government and its most representative institutions decided to release the monster early."
Earlier on Tuesday denied motions by US prosecutors to either reverse her decision or hold a hearing about the origin of property Posada could use to post his bond, set at $250,000.
Posada, currently held at the Otero County jail in New Mexico, cannot be released until the terms of his bond have been executed.
If freed, he would be held under house arrest in Miami with family members acting as custodians pending his immigration trial.
In a further development the government announced news conference in Havana with relatives of the airliner bombing victims was to be held on Wednesday morning.
Havana has accused Posada of involvement in a series of bombings inside Cuba in 1997, and of plotting to assassinate Castro in 2000 when he was arrested with a large cache of explosives in Panama.
According to reports, in 2005 he secretly entered the US and requested political asylum, and has been held since then on illegal immigration charges.
Out of the public eye following intestinal surgery last summer and a temporary handover of power to his broither, Raul, Castro has made his presence felt through written articles in recent weeks.
Prior to Tuesday's communiqué he twice criticised the Bush government's policy on biofuels in articles.